1. Field of the Invention
The present invention relates to a camera. More particularly the present invention relates to an auto focus apparatus and its movement thereof, as well as a method for using an encoder in a camera.
2. Description of the Related Art
A common camera typically includes a lens system for forming an image of a subject on a film or on a surface of an image sensor, and an image sensor for detecting the image, formed by the lens system, in the form of an electric signal. The film or the surface of an image sensor is constructed to correspond to an image surface of the lens system. A focus position of the lens system changes according to a distance between a lens and a subject. Accordingly, only when a quantity of change of the image surface position according to a position of a subject falls within a range of depth of focus of the camera, can a picture having a good quality be photographed. In other words, in order to acquire clear images, a light-receiving surface of the image sensor must be located within the range of the depth of focus of the lens system.
Accordingly, an apparatus must be provided for the camera, which enables the position of a lens to be moved according to a distance between a subject and a common camera, especially for a camera having a macro function having a large amount of change of a focus position according to the change of a distance from a subject. More particularly, a camera having a macro function having a large amount of change in focus position such as a function for close-up photographing. A camera having a means for automatically adjusting a position relative to a subject is referred to in the art as an Auto Focus (AF) camera.
In such an AF camera, methods for judging an exact focus interval between a subject located at a specific distance and a lens include a method for measuring the distance between the camera and the subject, and a method for estimating a focus position by analyzing a preview image. Recently manufactured compact digital cameras generally use the latter method. A method for estimating a focus position by analyzing a preview image will now be described herein below with reference to a block diagram shown in FIG. 1.
FIG. 1 schematically illustrates an internal structure of a conventional AF camera. A lens unit 110, an image sensor 120, a driving unit 130, an image signal processor (ISP) 140, a display unit 150 and a control unit 160 are included as internal components of the conventional AF camera.
Still referring to FIG. 1, the lens unit 110 optically receives an image of a subject, and includes at least one lens 112. The image sensor 120 converts the image of a subject which has been optically received by the lens unit 110 into electric signals. The ISP 140 processes the electric signals input from the image sensor 120 in units of frames, and outputs the image frame which has been converted in such a manner as to be appropriate to the screen characteristic (i.e., size, image quality, resolution, etc.) of the display unit 150. The display unit 150 displays the image frames which have been input from the ISP 140, on the screen. The driving unit 130 moves the lens unit 110 according to the control of the control unit 160, and includes a motor (M) 132 providing a driving force and a carrier 134 moving the lens unit 110 forward and backward by the driving force. The control unit 160 controls the driving unit 130 and moves the lens unit 110 to the focus position.
An auto focus process for estimating a focus position by analyzing a preview image through use of the components of FIG. 1 mentioned above will now be described herein below.
First, the lens unit 110 is moved to a starting point. A subject is photographed at the starting point, and then an image frame is formed by the image sensor 120. An edge value, which has usually been set at the central part of a screen, within an AF window is extracted from the image frame, and then the focus characteristic of the starting point is detected. Herein, an “edge” corresponds to the contour of the subject and to a boundary in which the brightness on the image frame rapidly changes. The edge value represents the difference brightness of such an “edge”. The brightness of each pixel of the image sensor 120 is calculated and a standard value is compared with the brightness difference between two adjacent pixels with respect to the row and column directions of the image sensor 120, and then it is determined whether the boundary between the two pixels is the edge. Such an edge value is calculated by accumulatively adding the brightness differences of the pairs of pixels corresponding to the edge.
After calculating the edge value, there is an identification as to whether or not the location of the lens unit 110 corresponds to an end point. When the lens unit 110 is not located at the end point, the lens unit 110 is subsequently moved to the next position, and then the operations mentioned above are repeatedly performed in succession. When the lens unit 110 is located at the end point, the maximum edge value among the edge values resulting from the above-mentioned repetitive processes is determined. Then, the lens unit 110 is moved to a position corresponding to the maximum edge value, and the AF process is completed. Consequently, the subject can then be photographed in a state where the focus has been automatically adjusted.
With regard to the AF performance processing described above, the motor of the driving unit provides movement of the position of a lens in accordance with commands from the control unit 160. The types of motors used in the driving unit include a Voice Coil Motor (VCM) and a Piezo Linear Motor (PLM), etc. The VCM is main type of motor used for AF. The VCM is advantageous to providing accurate linear motion because of its quick response characteristic, and the VCM is advantageous for miniaturization and precise location control owing to its relatively long stroke distance. The VCM operates in such a manner as to change the position of the lens by applying a current to the coil of the VCM having a characteristic as mentioned above.
In such a VCM, currents for driving each VCM commonly have slightly different values. For the purpose of an exact AF, though it is the most ideal to manufacture an AF camera that suitably corresponds to each different VCM driving current value, such a method often requires a large amount of cost and time in an attempt to provide a desired result. For that reason, there arises a problem in increasing the manufacturing yield.
In order to increase the manufacturing yield, manufacturers will apply the minimum current value among VCM driving current values to the VCM within the range established in the manufacturing process, instead of manufacturing an AF camera in accordance with each corresponding individual VCM driving current value. Through the process mentioned above, one VCM driving current value is applied to VCMs having different driving current values that does not result in an optimum design. As each VCM has no current value optimized thereto, an AF step not used for the actual AF driving is generated according to the AF camera module, which produces an increase in the AF driving time. An example of the description is illustrated in the graph of FIG. 2 below.
FIG. 2 illustrates the position of the lens unit which has moved to the point which is equivalent to each current value when a specific driving current value for moving the lens is applied to a VCM in the AF camera module to which VCM driving current value established on the conventional manufacturing process is applied.
Referring to FIG. 2, since an A, i.e., a first AF start step, and an A+Δ, i.e., a second AF step, have the same position of lens (0), an AF time is delayed by 1 step. That is, the VCM cannot have an optimized driving current value because of the VCM driving current value established on the manufacturing process and thus, it is noted that the A, i.e., the first AF start step is an unnecessary step not used for actual AF driving. Since the lens usually returns to the originally set position after AF, the unnecessary step mentioned above is continuously performed.
The result of the unnecessary step mentioned above is that there occurs a problem that time required for searching the maximum edge value increases because of the need to perform an unused step. In order to address the problem, there is has been a long felt need in the art for an AF method guaranteeing the rapid AF operation characteristic of an AF camera regardless of the VCM driving current value of the motor of individual driving unit.